To address the many serious problems caused by the use of carbon-based fuels, much attention is now being focused on the use of hydrogen as a non-polluting fuel. It is known that hydrogen gas (H2) can be produced from many different feedstocks such as natural gas, biomass, or water using a number of different techniques such as reformation, water gas shift reaction, gasification, or electrolysis. Several known methods include steam methane reformation (SMR), coal gasification, non-catalytic partial oxidation, biomass gasification and pyrolysis, and water electrolysis.
Steam methane reformation (SMR) has received a good deal of attention because of a belief that it can be used in economical and commercially viable processes. The feedstock is typically natural gas and the process reacts methane (CH4) with steam (H2O) to form a gas stream that includes H2 and CO. The CO is further converted to CO2 using the water gas shift reaction, liberating further H2. The CO2 must be separated from the gas stream to form pure H2.
Hydrogen production from coal gasification is another established technology. In the coal gasification process, steam and oxygen are utilized in a coal gasifier to produce a hydrogen-rich gas. Relatively high purity hydrogen can then be created from the synthesis gas by a water gas shift reaction and removed with separations processes. Other gases such as fuel gases and acid gases must also be separated from the hydrogen. Hydrogen can be similarly formed by the gasification of liquid hydrocarbons such as residual oil.
The manufacture of hydrogen by the reduction of steam using a metal species is also known. For example, U.S. Pat. No. 4,343,624 to Belke et al. discloses a three-stage hydrogen production method and apparatus utilizing a steam oxidation process. In the first stage, a low Btu gas containing H2 and CO is formed from a feedstock such as coal. The low Btu gas is then reacted in a second stage with ferric oxide (Fe3O4) to form iron (Fe), carbon dioxide (CO2) and steam (H2O) in accordance with the reaction:Fe3O4+2H2+2CO→3Fe+2CO2+2H2OThe steam and iron are then reacted in a third stage to form hydrogen gas by the reaction:3Fe+4H2O→Fe3O4+4H2 The iron oxide is taught by the patentee to be recyclable to the second stage for use in the iron oxide reduction reaction, such as by continuously returning the iron oxide to the second stage reactor via a feed conduit. At least one of the stages takes place in a rotating fluidized bed reactor. However, in actual practice, it is difficult to maintain the redox activity of pure iron oxide over multiple reduction/oxidation cycles. Furthermore, the co-current gas/solid contacting pattern is expected to restrict conversion efficiencies.
U.S. Pat. Nos. 5,827,496, 6,007,699, and 6,667,022 describe a method and apparatus for separating synthesis gas (a gas comprising mainly H2 and CO) and other gaseous fuels into separate streams of wet H2 and CO/CO2 using a mixture of limestone and iron oxide circulating between two fluidized bed reactors. U.S. Pat. No. 6,669,917 describes a similar process using a set of three fluidized beds reactors.
U.S. Pat. Nos. 6,663,681, 6,685,754, and 6,682,714 are all directed to a method of producing H2 gas using low cost carbon feedstocks, including high sulfur coal, and steam. The two-step process injects into a molten metal (Fe) bath reactor. The oxygen in the steam reacts with the iron to form H2 and FeO. In the second step, carbon fuel is inputted, the FeO is reduced to its metallic state, and CO2 is released. However, the process must be carried out at very high temperatures above about 1100°-1300° C. in specially-designed ceramic reactors such as those used in smelting operations. U.S. Pat. No. 5,447,024, teaches a chemical looping combustion method for a power plant that includes reacting a hydrocarbon fuel with a metallic oxide in a first reactor to release gases containing carbon dioxide and water vapor to operate a turbine while reducing a portion of the metal oxide. The reduced metal oxide is reacted in a second reactor where it is oxidized with air, to produce a second gas stream that is also used to operate a turbine. A small amount of steam is added to the air primarily to improve the heat transfer inside the bed, rather than to produce any hydrogen. In one embodiment, the metal oxide is a nickel oxide admixed with yttrium-stabilized zirconium and then sintered at high temperatures to form solid, non-porous particles. It is taught that the particles may be recycled between the two reactors.
However, despite the amount of research taking place, an economically viable process for producing large amounts of relatively pure hydrogen gas remains elusive. Thus, there remains a need in this art for such a process.